The SATA Problem
Before we talk about what mSATA is, let's talk about why SATA is becoming a problem.
SATA is a method of connecting a hard drive to a computer. It's an acronym for Serial Advanced Technology Attachment, although that underlying meaning is so rarely referenced today that it's arguably no longer relevant. This connectivity option was developed as an industry-wide standard capable of handling the high bandwidth potential of new hard drives. Another important advancement was a reduction in size. The common previous solution, called PATA, used a massive ribbon cable.
The modern SATA connection seemed quite tiny at the time it was developed, it has had a good run. However, as small laptops and tablet computers become more prevelant, companies are looking to introduce hard drives even smaller than the current 1.8" and 2.5" models found in many devices.
Many companies, such as Apple, have decided to get around the size restrictions of SATA and SATA compatible drives by soldering memory chips directly on their device's hardware. This saves space, but it also reduces future upgrade options and increases the cost of repairs. This leaves companies with a problem of compromise where neither option is ideal - they can use their own solution to connect long-term memory to the device, thus decreasing compatability and increasing repair costs, or they can use SATA and make concessions in their design.